The present invention relates to a method of starting up turbines and, more particularly, to a method of starting up turbines in a minimum length of time without causing thermal stress in the turbine to exceed a predetermined limit during acceleration of the turbine or during the controlling of load on the turbine.
In general, in the course of acceleration and load control of the steam turbine, attention must be paid above all to thermal stresses generated in the turbine since thermal stresses are strictly limited from a view point of safety.
According to the recent technique of managing the turbine system, the thermal stresses are severely controlled to prolong the life of the turbine which may be shortened at each time of starting.
The thermal stresses generated in the turbine are closely related to the acceleration rate or to the rate of change in load and generally tend to become larger as the acceleration rate becomes larger.
Therefore, in order to restrain the thermal stresses within a predetermined range, the acceleration rate has become one of the serious considerations.
In general, in starting up the turbine, rolling-up and warming are effected alternatingly. Thus, thermal stresses are related also to the duration of warming. More specifically, thermal stresses are increased during acceleration and then are gradually decreased during the subsequent warming. The larger the thermal stresses, the longer the duration of warming.
Conventional thermal power plants have been normally run at their base loads to meet the requirements of the systems connected thereto, and have been kept going for several months without suspension once they are started. In this case, the turbine can be smoothly started so long as attention is paid to thermal stresses. Accordingly, the operation of the turbine is performed at a sufficiently slow acceleration rate with a sufficiently long length of time.
One the other hand, recent thermal power plants, in particular, those of small or medium scale of capacity have predominated which are operated with frequent cyclic starting and stopping. For example, according to a mode of operation called "Daily Start and Stop", boilers are set on at 5 A.M. every morning and the start-up of the turbines is completed at 8 A.M. The turbine is then operated whole through the day time until 10 P.M. This cyclic starting-up and stopping of the plant are repeated every day. In another mode called "Weekly Start and Stop", the plant is started up, for example, at 8 A.M. on Monday and works whole through the week until it is stopped at 10 P.M. on Saturday, which cyclic operation is repeated every week.
In these cases, it becomes important to minimize the length of time required for the starting-up, although the restraint of the thermal stress is of the ultimate importance.
For instance, supposing that a plant in which the starting-up must be completed at 8 A.M. fails to start at that time, disturbance due to shortage of power may be resulted in the system concerned. This disturbance can be overcome only through increasing the outputs of other plant or plants. Thus, it is strictly required that the plants must be at latest started by the time expected, e.g. at 8 A.M. which in turn necessitates minimum length of time for starting-up the turbine. In addition, when a thermal power plant is tripped due to accidents in the system out of the plant, the plant must be reset as soon as the source of the trouble in the system is removed, in which case the plant including boilers and turbines which remain still warm must be restarted within a minimum length of time by so-called rapid restart.
Thus, recently, a method of starting up turbines have been longed for, which can simultaneously satisfy two contradictive requirements of minimizing the length of time for the starting-up and limitting thermal stresses within an allowable range.